Naoki Kamegashira

Phase equilibria in the system between NbO2 and Nb2O5 at high temperatures

Abstract Isothermal electrical conductivity measurements on niobium oxides were carried out over the temperature range from 1010 to 1300°C as a function of oxygen partial pressure in order to clarify the phase relations. Existence regions of the intermediate oxide phases between NbO2 and Nb2O5 were found from the discontinuities in electrical conductivity curves. These oxide phases were also analyzed by gravimetric method and by X-ray diffractometry. From these results the phase diagram for this system is proposed. The defect structures of these phases are also discussed.

SINGLE CRYSTALS OF URANIUM OXIDES BY CHEMICAL TRANSPORT REACTIONS.

Abstract Single crystals of various uranium oxides such as UO 2 , U 4 O 9 and U 3 O 8 were prepared by chemical transport reactions using halogen gases as transporting agents with a temperature gradient of 1000°C to 850°C in closed tubes.

Synthesis of single crystals of nonstoichiometric uranium oxides by chemical transport reactions

Single crystals of UO2, U4O9 and U3O8 were prepared by chemical transport reactions using HCl gas as the transporting agent. The O/U ratios of uranium oxides before and after transport were unchanged, and single crystals of the nonstoichiometric compounds such as UO2+x and U4O9-y were successfully prepared. The transport rates of UO2 single crystals were almost proportional to the initial HCl gas pressures. The transport rates of the uranium oxides increased rapidly as the O/U ratio increased. The transport mechanism with emphasis on the control of nonstoichiometry is discussed. The importance of uranium oxychloride species during transport reactions was derived.

Vaporization of niobium dioxide by mass-effusion and mass-spectrometric methods

Abstract The congruence of the vaporization process of NbO, NbO 2 , Nb 12 O 29 and Nb 2 O 5 in the niobium-oxygen system was investigated from the phase change of the solid residue after vaporization, and it was observed that only the NbO 2 phase vaporizes congruently. The vapor pressures over NbO 2 (s) were measured by means of a combination of mass-effusion (weight loss measurement) and mass-spectrometric methods in the temperature range 1953–2323 K. By applying the second and the third law treatments of thermodynamics to the partial pressures of the gaseous species NbO 2 (g), NbO(g) and O(g), the enthalpies of vaporization for the reactions NbO 2 ( s ,1) = NbO 2 ( g ) and NbO 2 ( s ,1) = NbO ( g ) + O ( g ), were calculated. From these data the enthalpies of formation and the dissociation energies of NbO 2 (g) and NbO(g) were also determined. The uncertainties included in the third law treatment were discussed, and the results calculated by the third law treatment using the most reliable data available at present were presented.

Growth of single crystals of U1−xThxO2 solid solutions by chemical transport reactions

Abstract Single crystals of U1−xThxO2 solid solution (x up to 0.69) were synthesized by closed-tube chemical transport using HCl gas as a transport agent. Two source materials were used: (1) solid solutions and (2) two-phase mixtures of UO2 and ThO2. The compositional ratios Th/U in the source materials against those of the crystals produced were measured. The larger values of x could be obtained when solid solutions were used as source materials rather than mixtures of UO2 and ThO2. The dimensions of the crystals produced were about 1 × 1 × 1 mm3 and the colors changed from black in uranium-rich crystals to reddish brown in thorium-rich crystals.

Synthesis of nonstoichiometric zirconium carbide whiskers by chemical vapor deposition

Abstract Nonstoichiometric zirconium carbide crystals with various compositions were prepared by chemical vapor deposition. Two gaseous mixtures, zirconium tetrachloride and argon, toluene and hydrogen, were introduced to the reaction zone where a graphite substrate was heated between 1200 and 1400°C. The deposition rate was proportional to the partial pressure of toluene. The compositional ratio of n C / n Zr in the gaseous mixture from 2.0 to 6.0 was found to be optimum for producing needle-like crystals. Needle-like crystal with smaller size were formed when the ratio of n C / n Zr was smaller than 2.0, and less needle-like crystals accompanied with more carbon were also produced when the ratio of n C / n Zr was larger than 6.0. The temperature of the substrate suitable for the growth of needle crystals was in the range from 1250 to 1300°C. The lattice constants of the products varied as a function of the ratio of n C / n Zr in the gaseous mixtures.

Electrical conductivity and defect structure of nonstoichiometric Th3N4 and Th2N2O

Abstract The temperature dependence of the electrical conductivity of Th3N4 has been measured in the temperature range 1000–1353 K, which reveals that Th3N4 is a semiconductor with an activation energy of 1.40 eV. The electrical conductivities of nonstoichiometric Th3N4 and Th2N2O have been measured as a function of nitrogen partial pressure in the temperature range 1273–1473 and 1323–1423 K, respectively. From the pressure dependence of the electrical conducitivity it has been found that both Th3N4 and Th2N2O are n-type semiconductors, and the predominant defects of the nonstoichiometric phases are thought to be triply ionized nitrogen vacancies

HIGH TEMPERATURE CHEMISTRY OF CERAMIC NUCLEAR FUELS WITH EMPHASIS ON NONSTOICHIOMETRY

Publisher Summary The choice of ceramic materials for nuclear fuel is mainly caused by their favorable properties at high temperatures. These high-temperature properties of the fuel material strongly influence the performance of a nuclear power reactor. This chapter reviews the phase equilibrium of these ceramic nuclear fuels at high temperatures and describes their nonstoichiometric region, defect structure, and thermodynamic data. The composition of a nonstoichiometric compound varies continuously with change of external conditions, such as temperature and pressure, and the structure, in turn, varies continuously with the composition. A continuous change of structure is obtained by adding a defect structure continuously to the structure of stoichiometric composition. To identify the different defect structures, the following classification is used: (1) simple (point) defect, (2) complex defect, and (3) stacking fault. Diffusion and vaporization processes are taken as representative phenomena characteristic of the ceramic materials for nuclear fuel at high temperatures. The chapter reviews these phenomena in their relation to nonstoichiometry.

Preparation of Plutonium Metal, (I):Preparation of Plutonium Metal by Lithium Reduction of Ammonium Plutonium(IV) Fluoride

A method of producing Pu metal by the reduction of sodium plutonium(IV) fluoride is reported. After the valency of Pu in acidic solution was adjusted to tetravalent state by addition of nitrite (e.g. NaNO2), sodium plutonium (IV) fluoride was precipitated by adding hydrofluoric acid in the presence of sodium ion in the solution. The double salt obtained by the wet method was dried under reduced pressure at 110°C, and then reduced with Ca metal in Ar atmosphere. Over 80% Pu yield was obtained by reducing 1 g batch of the double salt.